1. Description of the Related Art
The present invention relates systems for generating and focusing x-ray radiation for analytical instruments including x-ray diffractometry, x-ray spectrometry or other x-ray analysis applications.
2. Description of the Known Technology
There are numerous analytical instruments and procedures for which x-ray radiation is directed onto a target for analytical or metrology applications. Examples of such instruments include those based on the principles of x-ray coherent scattering such as x-ray scattering and x-ray diffraction, and those based on the principle of x-ray fluorescence such as x-ray spectroscopy and x-ray elemental mapping microscopy. In many such applications, there is a need to direct an intense beam of x-rays having controlled beam characteristics in its interaction with the target. These characteristics include spatial definition (divergence, beam size, focal spot size and intensity distribution at different locations), spectrum purity and intensity. However, these characteristic parameters can not be optimized independently. Improving one often comes at price of others. X-rays are inherently difficult to direct. Different technologies have been employed to form x-ray beams. These include total reflection reflectors, optics based on total reflection principle such as capillary and polycapillary made of bundle of micro-sized waveguides, natural crystals, and man-made layered structures called multilayer optics. In some cases, polychromatic radiation with energy spectrum over a relatively wide range may be desired. In other applications, highly monochromitized radiation is desired. Optics are made with selected technologies to match with the beam requirements while maintain an acceptable cost.
X-ray beam systems with excellent performance have been developed with microfocusing sources and variety of beam conditioning optics. Typical focal spot projection of these microfocusing sources is less than 100 micrometers and as small as 10 micrometers. Future development of source technology and optics technology may drive the brilliance even higher and spot size even smaller. Both stability of the spot size and spot position are critical for x-ray beams in analytical applications. In addition to superior performance, microfocusing sources use much less energy therefore has a lower operation cost and cause less environment issues. Sealed tube microfocusing sources, not only offers good performance, but also offers good performance-cost ratio. Representative optics in a microfocusing sources based beam system include multilayer optics, crystal optics, total reflection mirrors, mono-capillary optics and polycapillary optics. Optics can be designed for redirecting x-rays in one direction only, i.e. so-called one-dimensional optics (1D optics), or designed for redirecting x-rays in two perpendicular directions either through single interactions, two interactions or multiple interactions, i.e. so called two-dimensional optics (2D optics). For a highly intense beam, close coupling to an x-ray source is critical in order to acquire a large solid capture angle. To obtain a monochromatic beam, diffraction element should be a key part of the system.
Multilayer optics naturally delivers monochromatic beams. The beam characteristics, such as spatial definition, spectrum purity and intensity, can be optimized through various designs. Multilayer optics have been the major beam conditioning optics for x-ray scattering and diffraction.
In many analyses, such as in x-ray powder diffraction and thin film analysis, the probe beam is conditioned typically by a one-dimensional optic, meaning to redirect and form a beam in one direction only. These optics include planar multilayer optic, parabolic multilayer optic, and elliptical multilayer optic. These optics have a profile of cylinder curve, i.e. the curvature in the direction perpendicular the beam propagation direction is straight line, and the curvature in the direction of beam propagation direction is a profile of either straight line (planar optic), or part of a parabola (collimating optics), or part of an ellipse (focusing optics). These optics are typically very efficient and are capable in delivering high flux beams.
For many other applications, such as single crystal crystallography represented by small molecule crystallography and macro molecule crystallography (protein crystallography), the probe beam has to be a two-dimensional beam, i.e. a “pencil-like” beam formed in two perpendicular directions. Such a beam can be formed by a two-dimensional optic. Multilayer two-dimensional optics are the major beam conditioning optics for the need of two-dimensional beam conditioning. These optics delivers beams with well defined spatial characteristics and good spectrum purity.
Optics based on the waveguide principle, such as waveguide bundle optics represented by polycapillary optics, have been used in x-ray micro-spectrometry and selected x-ray diffraction applications. Comparing to multilayer optics, waveguide bundle optics offer much large capture angle and therefore potentially much higher flux and brilliance. The issue with waveguide bundle optics is that the output, in nature, is x-rays with continuous spectrum and is not suitable for x-ray elastic scattering and x-ray diffraction.
Being able to analyzing small sample is highly important, whether this is because of a local interest on a large sample or acquiring adequate signal strength from small available sample volume. High flux with well defined spectrum and spatial characteristics is often delivered by a focusing multilayer optic. Such an optic could consist of two cylinder elliptical mirrors; each of the mirrors focuses x-rays in one of the two perpendicular directions and the two mirrors are in a so-called Kirkpatrick-Baez geometry, either in sequential or “side-by-side” arrangement as depicted in U.S. Pat. No. 6,041,099. Such an optic could also be part of an ellipsoid with multilayer coating inside, where a single reflection from the optic directing the x-rays in 2-dimensions.
Further improving the intensity of a multilayer focusing optical system depends on close coupling between source and optic. Unfortunately, the coupling distance is limited by the physically feasible dimension of the structure at low d-spacing end. The smallest layer thickness of the man-made layer structure is limited by the size of atoms. At extremely low d-spacing end, such as lower than 10 angstroms, the inter-layer roughness is high; the peak reflectivity is low; and rocking curve is narrow.
As it can be seen, none of the solutions discussed above offers efficient coupling with a source and meanwhile provide a beam with controllable and satisfactory spectrum. U.S. Pat. No. 6,504,901 proposed an optical system coupled with a x-ray focusing mirror. But the proposed solution failed to demonstrate that the system will deliver a monochromatic beam and failed to illustrate its efficiency improvement. In fact, the description of the patent leads to a solution which is less efficient and renders an optical scheme without practical significance. The intention seems that using a polycapillary optic to form a small, intense and low divergence “virtual source”, the second optic, being a reflector limited with its capture angle, would be able to take the advantage of a source that is the small, more intense and with a lower divergence, and thus deliver a higher flux.
However, from physics law we know, that the first optic, the polycapillary optic, as a kinematical system, i.e. without energy input, will not be able to convert a beam with large divergence into a beam with lower divergence without enlarging the focal spot size of the virtual source. This can also be illustrated by applying thermodynamics to the optical system: the entropy, or the ordering represented by the spot size and divergence, of an isolated system without external energy input will at best be preserved and can not be reduced (or improved in terms of spot size and divergence). The description in U.S. Pat. No. 6,504,901 “polycapillary lens comprises a plurality of tapered capillaries arranged such that both the diameter of the focal spot of an x-ray source and the angular divergence of x-rays are reduced” inevitably results in, in the best case, the same brilliance. Therefore, the performance of the system, in the best case, is equivalent to the performance of the direct coupling between the second optic and the source.
The low efficiency of the proposed system in U.S. Pat. No. 6,504,901 could also be illustrated in an geometrical manner, as well. The mechanism of x-ray photons propagation through a single capillary is multiple external total reflection. It occurs in a quite small range of incident angles, which is below 0.3 degrees for the wave length commonly used for diffraction experiments. A collimating waveguide bundle optic, as depicted in FIG. 3, has a smaller cross section at a distance closer to the source. For an x-ray photon propagating inside a capillary, the incident angle at capillary wall gets smaller with each consecutive total reflection. On the other hand, if the optic is a focusing optic, the reflection angle gets smaller at first until the x-ray photons reach the point with largest diameter of the polycapillary optic, then gets larger with each consecutive reflection after passing the point with maximum diameter. When photons reach the exit of the proposed optic in the patent “bottle-shaped” optic where capillary diameter is smaller than at optic entrance, some portion of them will have incident angle larger than critical angle of external total reflection and will be lost, reducing optical system efficiency.